Durceler 19mm inserts (2pcs a Pack) to compatible with Medela / Spectra / Elvie / Rainyb / S18 S21 S23 / Momcozy S9 Pro S10 S12 Pro / TSRETE / Bellababy / NCVI 24 mm breast shields. It will reduce the 24mm shield nipple tunnel down to 19mm. Also compatible with Freemie 25mm cups to reduce it down to 19mm. Also applicable to most other brands of the models for 24-25mm. Seamless Compatibility For Optimal Milk Expression: Suitable for nipple diameter 16-18mm(after a pumping session), or measure the circumference of nipple base about 43.96 - 50.24mm before a pumping session. Great solution for small-nippled mothers using large breast shields, Proper nipple tunnel fit is essential for comfort and efficient milk output. It can be compatible with Spectra and Medela (Freestyle, Harmony, Sonata, and Swing-Maxi pumps) and other brands. BPA free, made of silicone material. Durable, clear, and comfortable. This accessory helps breast shields for more effective pumping and maximized milk expression while helping to avoid nipple trauma or irritation. Maintain compatibility with most handsfree bra. You can use most handsfree bra cups with they original 24/25mm detachable shields having this insert inside. During operation, you need to squeeze slightly to make the Insert fully fit with the shield to achieve the perfect effect. This product has a variety of models, which can change the size of the "breast milk channel" on the cover of the breast pump, so as to be suitable for mothers with different nipple sizes, help to improve the breast feeding efficiency and increase the pumping capacity, and make mothers more comfortable in the process of breast feeding.